1. Field of the Invention
This invention relates to a demodulator of amplitude-modulated signals transmitted from the output of a color video camera to apparatus such as a camera control unit (referred to as CCU hereinafter).
2. Description of the Prior Art
The transmission of video signals from the output of a video camera to a CCU is made along a cable while returning the video signal from the CCU over the same cable. Then, of the video signal from the video camera output, the luminance signal is amplitude-modulated onto a fixed frequency carrier wave, and two color difference signals are quadrature two-phase modulated before being transmitted. In the CCU, amplitude-demodulation is performed to construct the original luminance signal and two color difference signals.
Prior art demodulator circuitry for the amplitude-modulated signals accomplished the process consisting of passing the input signal through a band pass filter and a limiter to extract the carrier signal and supplying it to PLL circuitry, from which a carrier signal for use in demodulation is generated. It however has defects: the phase of output of the limiter depends on the input level, the phase of the carrier signal for synchronous demodulation is of low accuracy, and, because of the limited loop gain of PLL circuitry, there is a risk of unstability until the locked state is attained.